This invention relates to a liquid jetting apparatus such as an ink jet recording apparatus and a method of driving the same. Particularly, it relates to a liquid jetting apparatus for ejecting an extremely small amount of liquid droplet.
A related art will be discussed by taking an example an ink jet printer (a kind of ink jet recording apparatus), one form of a liquid jetting apparatus.
In the printer, the size of each dot on recording paper, namely, resolution is determined by the amount of ink droplet (a kind of liquid droplet) ejected through an ink jet recording head. Thus, it becomes important to control the ejecting amount of ink droplets. To make an attempt to control the ejecting amount by changing the caliber of a nozzle orifice, if the caliber is made small, the resolution is improved, but the recording speed becomes low; if the caliber is made large, the recording speed is increased, but a coarse image with a low resolution is formed. To satisfy such mutually contradictory requirements, the caliber of a nozzle orifice is sized large to deal with a large ink drop, and a waveform of a drive signal, namely, the way of driving the recording head, is devised so as to eject different amounts of ink droplets through the same nozzle orifice.
By the way, the improvement in image quality has been demanded for a recent ink jet printer. Thus, the waveform of a signal supplied to a piezoelectric vibrator for changing the volume of a pressure chamber is devised so as to eject an extremely small amount of ink droplet.
When an ink droplet is ejected, it is known that the ink droplet is separated into a main ink droplet and a satellite ink droplet associated with the main ink droplet. In an extremely small amount of ink droplet of about 4 pL (picoliters), the main ink droplet and the satellite ink droplet are of almost the same volume and each an amount of about 2 pL. A time lag exists between landing of the main ink droplet and the satellite ink droplet land on a recording medium. Namely, after the main ink droplet has landed on the recording medium, the satellite ink droplet lands thereon. Further, the jetting speed of the satellite ink droplet is lower than that of the main ink droplet. For example, the jetting speed of the main ink droplet is 7 to 8 m/s; while that of the satellite ink droplet is 3 to 4 m/s. Since the recoding head ejects the ink droplets while moving, there is probability that the landing position of the main ink droplet may shift largely from that of the satellite ink droplet.
It is therefore an object of the invention to eject an extremely small amount of droplet while decreasing the jetting speed difference between a main droplet and a satellite droplet.
In order to achieve the above object, according to the present invention, there is provided a liquid jetting apparatus, comprising:
a liquid jetting head, including a nozzle orifice, a pressure chamber communicated with the nozzle orifice, and a pressure generating element which varies the volume of the pressure chamber; and
a drive signal generator, which generates a drive signal including a drive pulse supplied to the pressure generating element, the drive pulse including:
a first expanding element, which drives the pressure generating element so as to expand the pressure chamber, so that a meniscus of liquid in the nozzle orifice is pulled toward the pressure chamber, the first expanding element being supplied for a time period which is not greater than a half a natural vibration period of the pressure chamber;
a first contracting element, which drives the pressure generating element so as to contract the pressure chamber expanded by the first expanding element, so that a center portion of the meniscus is swelled in an ejecting direction of a liquid drop, a potential difference of the first contracting element being not greater than 60% of a potential difference between a minimum potential and a maximum potential of the drive signal; and
a second expanding element, which drives the pressure generating element so as to expand the pressure chamber contracted by the first contracting element, so that a marginal portion of the swelled center portion of the meniscus is pulled toward the pressure chamber.
In this configuration, the amount of a liquid pillar generated in the center of the meniscus with supply of the first expanding element, the first contracting element, and the second expanding element can be extremely lessened, so that the ejected droplet amount can be decreased.
Further, since the supplying time period of the first expanding element is not greater than one half the natural vibration period of the pressure chamber for largely pulling in the meniscus, reaction of the pulled meniscus can be used so that desired pressure can be obtained even though the potential difference of the first contracting element is small (namely, with low voltage). Accordingly, since loads on the pressure generating element can be reduced, stable ejecting of a droplet and prolonging the life of the pressure generating element can be attained.
Preferably, a potential difference of the first expanding element is equal to the potential difference of the drive signal.
Preferably, the potential difference of the first contracting element is not greater than 50% of the potential the drive signal. Here, a potential difference of the second expanding element is not less than 40% of the potential difference of the drive signal.
Here, it is preferable that the potential difference of the second expanding element is not greater than the potential difference of the first contracting element.
Preferably, the second expanding element is supplied for a time period which is not greater than one quarter the natural vibration period of the pressure chamber.
Preferably, a gradient of the second expanding element is greater than a gradient of the first contracting element.
Preferably, the drive pulse includes a contracted state holding element, which connects the first contracting element and the second expanding element such that a termination end of the first contracting element and a start end of the second expanding element have an identical potential.
Here, the contracted state holding element is supplied for a time period which is not greater than one quarter the natural vibration period of the pressure chamber.
Preferably, the drive pulse includes a second contracting element, which drives the pressure generating element so as to contract the pressure chamber expanded by the second expanding element.
In this configuration, the meniscus moves in the ejecting direction as the second contracting element is supplied, and thus the liquid pillar is pushed from the root portion thereof. Thus, when the ink pillar is torn off and is separated into a main droplet and a satellite droplet to be jetted, the satellite droplet is urged by the meniscus so that the jetting speed of the satellite droplet can be increased. Consequently, the landing position of the main droplet can be matched with that of the satellite droplet.
Further, since the potential difference of the first contracting element is not greater than 60% of the potential difference of the drive signal, the termination potential of the second expanding element, which is the start end potential of the second contracting element, can be easily brought close to the termination potential of the first expanding element. Accordingly, the potential difference of the second contracting element can be easily enlarged. Consequently, the speed of the satellite droplet can be adjusted in a wide range without enlarging the potential difference of the drive signal.
Here, it is preferable that a potential difference of the second contracting element is not less than 75% of the potential difference of the drive signal.
In this configuration, the landing position of the main droplet and that of the satellite droplet can be brought closer to each other and the image quality can be further improved.
Here, it is preferable that the second contracting element is supplied for a time period which is not greater than one third of the natural vibration period of the pressure chamber.
Here, it is preferable that a time period from a start end of the first contracting element to a start end of the second contracting element is not greater than the natural vibration period of the pressure chamber.
Further, it is preferable that the time period between the start ends of the first contracting element and the second contracting element falls within a range of one quarter to one third of the natural vibration period of the pressure chamber.
Here, it is preferable that the drive pulse includes: a damping hold element, which holds a termination end potential of the second contracting element for a predetermined time period; and a damping element, supplied after the damping holding element to drive the pressure generating element so as to expand the pressure chamber to a reference volume thereof.
Further, it is preferable that the damping element is supplied for a time period which is not greater than a half the natural vibration period of the pressure chamber.
Still further, it is preferable that a time period from a start end of the first contacting element to a start end of the damping element is not greater than the natural vibration period of the pressure chamber.
Preferably, the drive pulse includes a preliminary contracting element, which drives the pressure generating element so as to contract the pressure chamber from a reference volume thereof, before the first expanding element is supplied.
According to the present invention, there is also provided a method of driving a liquid jetting apparatus provided with a liquid jetting head which includes a nozzle orifice, a pressure chamber communicated with the nozzle orifice, and a pressure generating element, the method comprising the steps of:
a first expanding step, for driving the pressure generating element so as to expand the pressure chamber, so that a meniscus of liquid in the nozzle orifice is pulled toward the pressure chamber as much as possible;
a first contracting step, for driving the pressure generating element so as to contract the pressure chamber expanded by the first expanding step, so that a center portion of the meniscus is swelled in an ejecting direction of a liquid drop;
a second expanding step, for driving the pressure generating element so as to expand the pressure chamber contracted by the first contracting step, so that a marginal portion of the swelled center portion of the meniscus is pulled toward the pressure chamber; and
a second contracting step, for driving the pressure generating element so as to contract the pressure chamber expanded by the second expanding step, so that the meniscus is again urged in the ejecting direction to increase jetting speed of a satellite liquid drop which follows a main liquid drop.
Preferably, the first expanding step is performed for a time period which is not greater than a half a natural vibration period of the pressure chamber.
Preferably, the second contracting step is performed for a time period which is not greater than one third of a natural vibration period of the pressure chamber.
Preferably, a time period between a time at which the first contracting step is started and a time at which the second contracting step is started is not greater than a natural vibration period of the pressure chamber.
Here, it is preferable that the time period between the start timings of the first contracting step and the second contracting step falls within a range of one quarter to one third the natural vibration period of the pressure chamber.
The invention can be embodied in various modes of a print method, a print apparatus, or the like.